The examination of fetal cells for early detection of fetal diseases and genetic abnormalities is undertaken in approximately one out of every thirty pregnant women. The main indication is maternal age (over 35 years). The tests may involve DNA gene typing or, more commonly, the use of live fetal cells for chromosomal karyotyping.
Fetal cells are usually obtained by amniocentesis, the removal of amniotic fluid from the amniotic cavity within the amniotic sac or placenta. The procedure presents a risk of harm to the fetus, particularly after the first trimester of pregnancy. The risk to the fetus together with the high cost of the procedure have prevented the establishment of examination of fetal cells for early detection of abnormalities as a routine procedure in pregnancy.
In the late 1970s and early 1980s, Herzenberg and his colleagues reported that fetal cells were present in maternal blood as early as 15 weeks gestation. The authors separated maternal and fetal cells using fluorescence-activated cell sorting (FACS) by staining maternal blood for a distinguishing paternal HLA antigen. The authors state that the demonstration that fetal cells enter maternal circulation and can be isolated by FACS-enrichment procedures could have practical significance in enabling karyotyping without the need for amniocentesis. The authors state that this would be possible if the frequency of successful isolation of cells at 15 weeks is sufficiently high and the cells could be induced to divide (enter metaphase). Furthermore, extensive HLA typing reagents, or other cell surface reagents would need to be developed to distinguish maternal and fetal cells. To date, the technique has not been successfully adapted for use as a clinical technique for either karyotyping or fetal DNA analysis.
Recently, fetal cells present in maternal blood have been used to perform analysis of genes present in the fetus. In one technique, the maternal and fetal cells were not separated and the DNA from the cell mixture was amplified with Y chromosome-specific primers to determine whether the fetus is male. It has been suggested that DNA amplification techniques can also be performed to detect gene sequences associated with disease in this manner. Of course, the method cannot be used where the mother is a carrier for the trait.
To date, amniotic fluid or chorionic villus biopsy has been the only source of antenatal cells to provide a sufficient number of live cells for karyotyping. Furthermore, DNA analysis methods have only been possible in relatively limited situations which depend on particular differences in maternal and fetal cells, e.g. presence of the Y chromosome in the fetus or presence of HLA-A2 antigen on fetal, but not maternal, cells.